Several publications are referenced in this application to describe the state of the art to which the invention pertains. Each of these publications is incorporated by reference herein.
Environmental stress is one of the most important limitations on plant productivity, growth and survival. An ever-increasing source of environmental stress to plants is the stress caused by environmental pollutants in the soil, water and atmosphere. Such pollutants include herbicides, pesticides and related agronomic products, as well as organic and inorganic waste material from industry and other sources. Other toxic agents that threaten the survival of plants include various toxins produced by ephiphytic or soilborne microorganisms, such as fungi and bacteria.
In order to survive in toxic environments, plants must have mechanisms to detoxify xenobiotics, heavy metals and other toxic compounds. This generally involves modification of the toxic compound and subsequent excretion into the vacuole or apoplastic space. Recently, certain ATP-binding cassette (ABC) transporters have been identified in plants, which appear to be involved in the detoxification process.
The ABC transporter family is very large, with representatives existing in many different classes of organisms. Two well studied groups of ABC transporters, encoded by mdr and mrp genes, respectively, are associated with the multi-drug resistance phenomenon observed in mammalian tumor cells. The mdr genes encode a family of P-glycoproteins that mediate the energy-dependent efflux of certain lipophilic drugs from cells. The mrp genes encode a family of transporters that mediate the extrusion of a variety of organic compounds after their conjugation with glutathione. YCF1, the yeast homolog of mrp, encodes a protein capable of glutathione-mediated detoxification of heavy metals.
Homologs of mrp and mdr genes have been identified in plant species. In Arabidopsis thaliana, the glutathione-conjugate transporter encoded by the mrp homolog is located in the vacuolar membrane and is responsible for sequestration of xenobiotics in the central vacuole (Tommasini et al., FEBS Lett. 411: 206-210, 1997; Li et al., Plant Physiol. 107: 1257-1268, 1995). An mdr-like gene (atpgp1) has also been identified in A. thaliana, which encodes a putative P-glycoprotein homolog. The atpgp1 gene was found to share significant sequence homology and structural organization with human mdr genes, and was expressed with particular abundance in inflorescence axes (Dudler & Hertig, J. Biol. Chem. 267: 5882-5888, 1992). Other MDR homologs have been found in potato (Wang et al., Plant Mol. Biol. 31: 683, 1996) and barley (Davies et al., Gene 199: 195, 1997).
The aforementioned mrp and mdr plant homologs were identified as a result of an effort to understand the molecular basis for development in plants of cross-resistance to herbicides of unrelated classes. However, these transporters are likely to serve the more general role in plants of sequestering, secreting, or otherwise detoxifying various organic and inorganic xenobiotics. Accordingly, it will constitute an advance in the art of plant genetic engineering of stress tolerance to identify and characterize other members of this class of transporters in plants.